Recently, the polygon mirror scanner motor is demanded to be smaller in size, thinner in thickness, and lower in cost along with the spreading use of the laser beam printer (LBP). At the same time, it is also requested to decrease the rotation fluctuation (jitter) and noise. Besides, maintenance of high precision is required for preventing plane tilting of the polygon mirror.
Conventionally, as shown in patent document 1, for example, it has been attempted to reduce the thickness and lower the cost by a structure of fitting a bearing to an iron base plate. The bearing has been enhanced in precision and extended in service life by using a fluid bearing, which is a kind of dynamic pressure bearing.
FIG. 4 is a sectional view of a polygon mirror scanner motor in conventional example 1 disclosed in patent document 1. In FIG. 4, rotor boss 402 is fixed to rotary shaft 401 by shrinkage fitting or other method. Polygonal rotating mirror 403 and rotor frame 404 are fixed to rotor boss 402. Rotor magnet 405 is fixed to the inner wall of rotor frame 404. Rotary shaft 401, rotor boss 402, polygonal rotating mirror 403, rotor frame 404, and rotor magnet 405 are combined to constitute rotor 400.
Stator base plate 411 of the polygon mirror scanner motor shown in FIG. 4 has a mounting portion for mounting the polygon mirror scanner motor on a device. Stator base plate 411 is formed of an iron base plate high in thermal conductivity.
Stator core 412 is formed by laminating magnetic members. Stator coil 413 is wound on stator core 412. Stator core 412 and stator coil 413 are combined to constitute winding assembly 414. Stator base plate 411 is provided with driving IC 415 for operating the polygon mirror scanner motor. Stator base plate 411, winding assembly 414, and driving IC 415 are combined to constitute stator assembly 410. Stator core 412 and rotor magnet 405 face each other across a gap.
Bearing 420 is inserted from the backside of stator base plate 411, and is directly crimped to stator core 412. In the inner wall of bearing 420, herringbone grooves are formed as dynamic pressure grooves, and bearing 420 composes a fluid bearing. Rotary shaft 401 is inserted into bearing 420, and bearing 420 supports to rotate rotary shaft 401.
When a current flows in stator coil 413, a rotary torque is generated between stator core 412 and rotor magnet 405. As a result, rotor 400 is put into rotation.
Along with rotation of rotor 400, polygonal rotating mirror 403 is also put into rotation. By rotation of polygonal rotating mirror 403, a wind is generated. By the cooling effect of this wind, the heat generated from bearing 420 is released from stator core 412 and stator base plate 411. As a result, the bearing performance is enhanced in the polygon mirror scanner motor shown in FIG. 4.
Further, by the cooling effect of the wind, the heat generated from driving IC 415 can be also released, and deterioration of the performance of driving IC 415 can be prevented.
The polygon mirror scanner motor disclosed in patent document 1 is a shaft rotation type, that is, rotary shaft 401 is supported and rotated by bearing 420. Other type is a polygon mirror scanner motor of shaft fixed type, that is, the bearing inserted to and supported by a fixed shaft rotates about the fixed shaft.
This polygon mirror scanner motor of shaft fixed type includes a plane opposed type motor in which a plurality of flat coils are disposed oppositely to a flat plate type rotor magnet. In this plane opposed type motor, it is attempted to reduce the thickness by integrally forming the flat plate rotor magnet, the rotor yoke, and the polygonal rotating mirror, for example, as proposed in patent document 2.
FIG. 5 is sectional view of a polygon mirror scanner motor in conventional example 2 disclosed in patent document 2. In FIG. 5, rotating polyhedron 510 has flat plate rotor magnet 511 and rotor yoke 512 disposed in its inside, and mirror surface 513 is formed on the outer circumference. A plurality of flat plate coils 521 are opposite to flat plate rotor magnet 511 across a gap, and are disposed on control base plate 522. Control base plate 522 is installed on mounting board 523 which serves also as bracket and back yoke.
In the center of mounting board 523, through-hole 524 is formed. Concave peripheral groove 526 formed on fixed shaft 525 is fitted to the peripheral edge of this through-hole 524, and fixed shaft 525 is held on mounting board 523.
In the center of rotating polyhedron 510, circular tube part 514 is formed. At both ends of circular tube part 514 in the axial direction, bearing 515 and bearing 516 are fitted. Circular tube part 514 is supported on fixed shaft 525 by way of bearing 515 and bearing 516.
In this construction, rotating polyhedron 510 is disposed nearly in the center of the axial direction of fixed shaft 525. Moreover, bearing 515 and bearing 516 are disposed at both ends of circular tube part 514, and the bearing is composed in two-side support structure. In this construction, the motor shown in FIG. 5 can rotate smoothly at high speed.
Along with the wide spread of the LBP, high speed and colorization of the LBP are demanded, and the polygon mirror scanner motor is demanded to increase the speed further from 30,000 to 50,000 min−1.
The bearing structure of the conventional polygon mirror scanner motor disclosed in patent document 1 is a so-called one-side support structure, and had a problem of “grinding motion.” In particular, in rotation at high speed from 30,000 to 50,000 min−1 the grinding motion gives serious effects on the dynamic pressure bearing, and the bearing life is shortened extremely. To solve this problem, the diameter of the rotary shaft must be increased to enhance the rigidity of the dynamic pressure bearing, but it causes other problems, such as increase of bearing loss, increase of power consumption, and increase in motor size.
The conventional polygon mirror scanner motor disclosed in patent document 2 is excellent in the bearing construction of two-side support structure. However, since the plane opposed type motor system is employed, when changing the energized phase, an attracting repulsive force is generated in the axial direction between flat plate rotor magnet 511 and flat plate coil 521. In particular, in rotation at high speed, large vibration or noise is generated.
Mirror surface 513 is formed on the outer circumference of rotating polyhedron 510 integrally forming circular tube part 514 accommodating bearing 515 and bearing 516, flat plate rotor magnet 511, and rotor yoke 512. Because of this construction, it is extremely difficult to form a mirror surface of high precision.
As the means for solving the problems, the following construction may be considered. First, the problem of large vibration or noise occurring in rotation at high speed may be solved by a structure in which the torque generating part of the conventional polygon mirror scanner motor disclosed in patent document 1, that is, the structure of stator assembly 410 and rotor 400 in FIG. 4 is replaced by the plane opposed type motor structure of patent document 2.
Next, in rotor boss 402 of patent document 1, a structure corresponding to circular tube part 514 of rotating polyhedron 510 of patent document 2 is formed. In the inner wall of this circular tube part, herringbone grooves are formed as dynamic pressure grooves, and a fluid bearing is composed together with the fixed shaft. Thus, by composing the shaft fixed type fluid bearing structure for rotating rotor boss 402 about the fixed shaft, a bearing structure close to the two-side support structure is realized. This construction solves the problems of large effects of grinding motion on the dynamic pressure bearing in rotation at high speed, and shortening of bearing life.
Moreover, same as in patent document 1, polygonal rotating mirror 403 (mirror) having mirror surface 513 isolated from rotating polyhedron 510 of patent document 2 is fixed to rotor boss 402. In this construction, polygonal rotating mirror 403 can be processed independently, and the problem of difficulty in manufacture of mirror surface of high precision can be solved.
Actually, a polygon mirror scanner motor having such construction is proposed, for example, in patent document 3. FIG. 6 is a sectional view of the polygon mirror scanner motor in conventional example 3.
In FIG. 6, annular protrusion 602 is formed on bracket 601. Stator core 603 is fixed to annular protrusion 602. Stator coil 604 is wound on stator core 603. Bracket 601 is mounted and fixed on iron plate circuit board 605. Fixed shaft 606 is pressed and fixed into the central part of bracket 601.
Hub 611 is provided with sleeve bearing 612 of circular tube shape projecting downward. Herringbone grooves are formed in the inner wall of sleeve bearing 612. By the herringbone grooves and a lubricant poured in a slight gap between fixed shaft 606 and sleeve bearing 612, a dynamic pressure is generated at the time of rotation of the motor. As a result, fixed shaft 606 supports sleeve bearing 612 rotatably.
Rotor 614 is mounted on outer wall 613 of sleeve bearing 612. Polygonal rotating mirror 615 of square shape is installed in the upper part of hub 611. Polygonal rotating mirror 615 is pressed and fixed from above by clamping spring 616.
However, in the polygon mirror scanner motor shown in FIG. 6, in the central part of bracket 601, the pressing and fixing part of fixed shaft 606 is formed on the backside of iron plate circuit board 605 by projecting largely. Accordingly, this polygon mirror scanner motor is hardly reduced in size and thickness.
Accordingly, in the polygon mirror scanner motor shown in FIG. 5, in the fixing structure of fixed shaft 525 and mounting board 523, there is no member projecting largely on the backside of mounting board 523, and it may be considered as means for solving the problem of reduction of size and thickness.
However, the polygon mirror scanner motor shown in FIG. 5 has a fixing structure of only fitting and holding the peripheral edge of through-hole 524 formed in the center of mounting board 523, to concave peripheral groove 526 formed in fixed shaft 525. It is hence difficult to ensure the verticality of fixed shaft 525 to mounting board 523, especially to keep precision of plane tilting of mirror surface 513, and the fixing strength is not sufficient.
Meanwhile, as means for fixing the shaft and the flat plate firmly without applying large external force, a method of crimping and fixing the shaft and the flat plate by using laser light is proposed. For example, the method disclosed in patent document 4 is considered as a method of solving the above problems.
FIG. 7A is an essential sectional view of crimping portion before laser light irradiation in the conventional crimping method using laser light, and FIG. 7B is an essential sectional view of crimping portion after laser light irradiation in the conventional crimping method using laser light.
In FIG. 7A, mounting hole 702 is formed in one member 701 of plate shape, and chamfering part 703 on its peripheral edge. On other member 711 of columnar shape, small end 712 fitted to mounting hole 702, and flange 713 of wide diameter are formed. After member 701 and member 711 are assembled, laser light 720 is emitted to end face 714 of small end 712. As a result, as shown in FIG. 7B, part 715 of small end 712 is melted, and is fluidized in the direction of chamfering part 703 of one member 701, and is solidified. By this fluidized part 715 and flange 713, other member 711 is completely solidified in one member 701, and is crimped in the axial direction.
In the conventional method, however, the flange is needed in order to fix the plate member and the columnar member at right angle, and the squareness depends on the precision of the flange and the plate member. Further, in the fluid bearing, an extremely high precision is required in the diameter crossing of the shaft corresponding to the columnar member and in the surface roughness, and when the shaft is formed on the flange, it is hard to satisfy the required precision.    Patent document 1: Japanese Unexamined Patent Application Publication No. H9-131032    Patent document 2: Japanese Unexamined Patent Application Publication No. H3-63617    Patent document 3: Japanese Unexamined Patent Application Publication No. H7-336970    Patent document 4: Japanese Unexamined Patent Application Publication No. S60-87987